The basic operation and structure of land mobile radio communication systems are known. Communication systems typically comprise a plurality of communication units (vehicle mounted or portable radios in a land mobile system and radio/telephones in a cellular system), a predetermined number of transceivers, which are located throughout a geographic region and transceive information via communication channels, and a controlling entity. A controlling entity may either be a centralized call processing controller or it may be a network of distributed controllers working together to establish communication paths for the communication units. The communication channels may be time division multiplex (TDM) slots, carrier frequencies, a pair of carrier frequencies or other radio frequency (RF) transmission medium. A frequency or time portion of one or more of the communication channels may be established for call control purposes such that a communication unit may communicate with the system controller to request and receive system resources.
In a trunked land mobile communication system, the plurality of communication units are arranged into communication groups, or talk groups. A communication unit in a particular talk group may initiate a dispatch call by pressing a push-to-talk (PTT) button which informs the controller that a communication channel is needed for this particular talk/group. If a communication channel is available, the controller allocates it to the particular talk group and sends out a message on the control channel to the plurality of communication units. The communication units in the particular talk group, after receiving the message, affiliate themselves with the allocated communication channel such that each member of the particular talk group can participate in the dispatch call. The number of communication units in any one dispatch call could range from just a few communication units to a few hundred communication units.
Multiple site communication systems which comprise a plurality of repeater sites over a large geographic region are known. In such systems, communication units of a particular talk group may be located anywhere in the multi-site coverage area. To establish a group call, the multi-site system must be able to quickly and efficiently set-up communication paths, or inter-site links, between all the sites, or between just those sites having a member of the particular talk group located within it. One method of establishing the communication links is simulcast. Simulcast uses the same communication channel (or carrier frequency) in each site for the particular group. This is an efficient frequency reuse technique when members of the particular group are routinely located throughout the multi-site system.
A typical transceiver in a simulcast multi-site communication system comprises an individual circuit that couples the repeater to the central radio system audio collection and distribution point (prime site). Each transceiver receives signals on the same frequency and transports the signals to the single audio collection point where a single signal comparator selects the best signal from all the sites. (Note that a site in the multi-site system may contain a transceiver (transmitter and receiver) or only a receiver.) The signal selected as the best is distributed from the centralized point on links back to the transceiver sites for simultaneous re-transmission. To accurately re-transmit the best signal, dedicated, stable, and time-invariant links are used. For example, the links may be analog and/or digital microwave channels. Note that switching systems are not used as links because they are not time-invariant.
With the dedicated, stable, and time invariant links, the site transmitters can re-broadcast the best signal in phase, in time, and on the same frequency such that received signal distortion in overlapping site coverage areas is minimal. The stability of the links ensure that the resulting simulcasted signals remain within acceptable tolerances.
To account for the difference in the physical link transport time delays between a single point of audio distribution and remote site transmitters, additional adjustable delay circuits are typically added to the links. The adjustable delay circuits compensate for the differences in physical link delay such that the total delay is the same at each transceiver site. Thus ensuring that the signal for transmission arrives at each transceiver site at the exact same time. The adjustable time delay devices added to the transmission distribution links may be at either the prime or remote sites.
To accommodate for fluctuations in physical link delays, means have been devised to manually or automatically adjust the adjustable time delay circuits. However, it is difficult for simulcast systems to adapt to time changes while user traffic is in progress. Typically, the channel must be excluded from service while a closed loop test is performed to measure and adjust the delay.
Many users of a simulcast system need immediate and constant access to their system channels. For these users disabling a channel for service is inconvenient at best and potentially catastrophic. Such is certainly the case for Public Safety users and centralized controller systems. If the centralized controller is cut off from the system due to a channel being down, communication units cannot communicate. To avoid this, some systems include duplicate prime site equipment. The duplicate equipment involves added logic and switching functions which slows the switch-over process.
Therefore, a need exists for a multi-site simulcast communication system that can efficiently utilize time-invariant or time-variant distribution links, be constructed without the delays of typical switching systems and that can instantly adapt to site failures and maintain the same constant grade of service while simulcasting transmissions.